The present invention is related generally to radio frequency (RF) filters and more specifically to coupling structures and methods for resonant sections in cavity filters.
It is well known that state of the art communications systems require high performance filtering devices in order to maximize performance and comply with federal communications laws and standards. These devices are formed to be highly frequency selective by minimizing signal loss within a desired passband and significantly attenuating unwanted signals which reside outside the passband.
A known method of forming a communications filter, such as a band pass filter, is to couple multiple resonant structures which reside within one or more tuned cavities. An example of such a filter is illustrated in U.S. Pat. No. 5,936,490 to Hershtig (the ""490 patent) which is directed to a filter formed with coupled tri-sections or coupled triplets. In the ""490 patent, a filter is formed with three high dielectric resonators placed within corresponding cylindrical cavities which are mutually coupled either through aperture coupling or probe coupling. In aperture coupling, a small window is presented between the cavities which provides a degree of magnetic coupling between adjacent cavities. The ""490 patent discloses that the amount of coupling can be adjusted by placing a tuning screw in the aperture. In probe coupling, a conductive probe, or dielectric probe is interposed between the resonant sections and is either in direct contact with the resonators or is tightly coupled thereto. The use of tuning screws is known to be problematic in that the manual adjustment is labor intensive to initially adjust and is not highly repeatable. In addition, once adjusted, the filter is subject to being detuned due to vibrations and/or thermal expansion and contraction which alter the screw position over time. Probe coupling can be problematic in that the probe must be carefully located with respect to the resonators and the tight coupling with respect to the resonator elements tends to detune the resonant elements and reduce the Q of the resonant elements.
Generally, the coupling elements for such filters may exhibit transmission line characteristics over a range of frequencies but at the frequency of interest (near the center of passband), they may be adequately described as lumped element inductors or capacitors, i.e., having a frequency response which described as either predominately inductive or capacitive. In the case of an inductive response, the element can be described as having a positive phase and for the capacitive response, the element can be described as presenting a negative phase.
As described in connection with the ""490 patent, coupled triplets (ct) have three resonant elements. This filter section has an asymmetrical response, producing a single transmission zero above or below the passband depending upon the relative phase of the coupling elements. Referring to FIG. 1A , when all of the coupling elements, including the bridging element (1,3) are phase positive, this produces a zero of transmission above the passband. Alternatively if the bridging element has a negative phase, as illustrated in FIG. 1B, this produces a zero of transmission below the passband In each case, the transfer function produces an asymmetrical response, which is desired for many communication system requirements.
In addition to coupled triplets, filters can also be formed as coupled quadruplets which have tour resonant elements, as illustrated in FIG. 1C. This filter section has a symmetrical response, producing zeroes of transmission which are distributed on both sides of the passband.
Generally, when an optimal transfer function is translated into a realizable device, the performance of the resulting filter will be a function of both the realization of the resonant sections and the coupling structures. Thus, not only is it important to use high Q resonant structures, but care should be taken in the design and implementation of the coupling structures to insure that the device is easily manufacturable and lends itself to a cost effective, repeatable, high performance device. These desirable features generally preclude, or limit, the use of tuning adjustments when ever possible.
Accordingly, it is an object of the present invention to provide a coupling structure that will provide either substantially magnetic coupling or substantially electric coupling between resonant elements.
It is another objective of the present invention to provide a coupling structure that does not substantially effect the quality factors of the resonant elements being coupled.
It is a further objective of the present invention to provide a coupling structure that may be used to suppress spurious passbands of a device that incorporates the aforementioned elements.
It is yet another object of the present invention to provide a coupling structure which is readily manufacturable and results in a reliable and repeatable filtering section.
In accordance with one embodiment of the present invention, a coupled filter section is formed having a plurality of cavity sections which are arranged such that at least a portion of the cavity sections are adjacent. The adjacent cavity sections are in magnetic communication via a waveguide channel. A plurality of resonators are disposed within the cavity sections. A substantially rectangular boss is disposed in at least one of the waveguide channels. The rectangular boss has a length, a height and a width which can be selected to provide a predetermined level of coupling between adjacent resonators.
The filter section can be formed with three cavities and three resonators in a coupled triplet configuration. In this case each resonator is adjacent to the other two resonators. Alternatively, the filter section can be formed with four cavities and four resonators in a coupled quadruplet configuration. In this later case, each resonator is adjacent to two of the other resonators.
Preferably, a coupled filter section is formed with a two piece construction which includes a first portion formed as a first unitary piece having relief areas defining a plurality of cavities, a corresponding plurality of resonators located within each of the plurality of cavities, a plurality of channels allowing magnetic coupling between adjacent cavities, and a plurality of coupling stubs disposed within the channels. The filter section also includes a second portion for affixing the first portion to substantially enclose the relief areas of the first portion.
Also in accordance with the present invention is a method of manufacturing a coupled filter section. The method includes forming a first portion from a first unitary piece by establishing relief areas therein which define a plurality of cavities, a corresponding plurality of resonators located within each of the plurality of cavities, a plurality of channels allowing magnetic coupling between adjacent cavities, and a plurality of coupling stubs disposed within the channels. The method further includes forming a second portion for cooperatively engaging the first portion to substantially enclose the relief areas of the first portion and affixing the second portion to the first portion.
Preferably, the method includes altering the size of the relief areas such that the dimensions of the resonators change, thereby altering a passband of the filter section. The method can also include altering the size of the relief areas such that the dimensions of the coupling stubs change, thereby adjusting the coupling between adjacent resonators.
The method can also include forming the second portion with at least one post member extending from the second portion. The post members are formed in a position to be in substantial alignment with at least one of the resonators after the first and second portions are affixed. The method can also include adjusting the height of the posts to adjust the characteristics of the filter section.
In one embodiment, the relief areas include a plurality of annular relief areas which simultaneously define both the cavities and at least a portion of the resonators. In addition, the relief areas defining each channel can include first and second relief areas which are substantially parallel thereto and define a substantially rectangular channel with a substantially rectangular coupling stub therein.